Frequency Management

2.  The function of frequency management is to divide
the total number of available channels into subsets
which can be assigned to each cell either in a fixed
fashion or dynamically.
 Frequency management refers to designating
set-up channels and voice channels, numbering
the channels, and grouping the voice channels into
subsets.
 Channel assignment refers to the allocation of
specific channels to cell sites and mobile units.
 Allocation of specific channels to cell site on long
term basis is called ‘fixed channel assignment’.
 Allocation of specific channels to cell site on short
term basis is called ‘dynamic channel assignment’.

4.  The total number of channels at present (January
1988) is 832. But most mobile units and systems
are still operating on 666 channels.
 A channel consists of two frequency channel
bandwidths, one in the low band and one in the
high band.
 For example frequencies of channel 1 are
825.03MHz(mobile transmit) and 870.03MHz(cell
site transmit)
 Two frequencies of channel 666 in AMPS systems
are
844.98MHz(mobile transmit) and 889.98MHz(cell
site transmit)

5.  Total available spectrum in AMPS (Advanced
Mobile Phone Systems) is 40MHz.
 40MHz Band Width is divided into 666
channels.
 Each channel has a bandwidth of 60KHz i.e.,
40MHz/666 which is approximately equal to
60KHz.
 Each channel means Duplex channel(30KHz
for forward channel and 30KHz for reverse
channel).

6.  The 666 channels are divided into two groups:
block A system and block B system. Each market
(i.e., each city) has two systems. Each block has
333 channels, as shown in Fig.
 The 42 set-up channels are assigned as follows.
Channels 313-333 block A
Channels 334-354 block B
 The voice channels are assigned as follows.
Channels 1-312 (312 voice channels) block A
Channels 355-666 (312 voice channels) block B
 These 42 set-up channels are assigned in the
middle of all the assigned channels to facilitate
scanning of those channels by frequency
synthesizers.

8.  FCC(Federal Communication Commission)
allocated new additional spectrum of 10MHz.
 10MHz means an additional
of166(10MHz/60KHz) channels are assigned.
 Out of 166, 83 channels are assigned to block A
and 83 are assigned to block B system.
 Since a 1MHz is assigned below 825 MHz(or
870MHz), in future, additional channels will be
numbered up to 849 MHz(or 894 MHz) and will
then circle back.
 The last channel number is 1023.
 There are no channels between channels 799 and
991.

10.  The number of voice channels in each system is
312. We can group these into any number of
subsets.
 Number of setup channels in each block is 21. So
it is logical to group these 312 channels into 21
subsets.
 Each subset has 16 channels.
 In each set, the closest adjacent channel is 21
channels away .
 In a seven cell frequency reuse cell system, each
cell contains three subsets i.e, iA+iB+iC, where i
is an integer from 1 to 7.
 The minimum separation between three subsets
is 7 channels.

11.  Set-up channels, also called control channels,
are the channels designated to set-up calls.
 Without set up channel also, system can be
operated.
 If we are choosing such a system then all 333
channels in each system(block A and block B)
can be voice channels
 However each mobile unit must then scan
333 channels continuously and detect the
signaling for its call.
 A customer who wants to initiate a call must
scan all the channels and find an idle
(unoccupied) one to use.

12.  In each block of system we have 21 setup channels.
 The number 21 is derived from a seven cell
frequency reuse pattern with three 120 degree
sectors per cell, or a total of 21 sectors, which
require 21 setup channels.
 Set up channels are classified into
1. Access channels 2.Paging channels
 An access channel is used for mobile originating
calls and paging channel is used for land originating
calls
 In a low traffic areas access and paging channels are
same
 Every two way channel contains two 30KHz
bandwidths.
 A set up channel also consists of two simplex
channels one is forward set up channel(BS to MS)
and another one is reverse set up channel(MS to BS).

13.  In the most common types of cellular systems,
one set-up channel is used for both paging and
access .
 The forward set-up channel functions as a
paging channel for responding to the mobile
originated calls.
 The reverse set-up channel functions as the
access channel for the responder to the paging
call.
 The forward set-up channel is transmitted at
cell-site and reverse set-up channel is
transmitted at mobile unit.
 All the set-up channels carry data information
only.

14.  Generally Access channels are used for mobile
originating calls.
 Access channels carries information from mobile unit
to cell site.
 In mobile- originating calls, the mobile unit scans its
21 setup channels and chooses the strongest one.
 Because each set-up channel is associated with one
cell, the strongest set-up channel indicates which cell
is to serve the mobile originating calls.
 The mobile unit detects the system information
transmitted from the cell site.
 Also the mobile unit monitors the Busy/Idle status
bits over the desired forward set-up channel
 When the Idle bits are received, the mobile unit can
use the corresponding reverse set-up channel to
initiate a call.

15.  Operational functions
◦ 1.Power of forward setup channel: The power of the set-
up channel can be varied in order to control the number
of incoming calls served by the cell. When the traffic is
high most voice channels are occupied and the power of
the set-up channel should be reduced in order to reduce
the coverage of the cell for the incoming calls
originating from the mobile unit.
◦ 2.The set-up channel received level: If the received
power level is greater than the given set-up threshold
level, call request will be taken.
◦ 3.Direct call retry: When a cell site has no available voice
channels, it can send a direct call –retry message
through the set-up channel.

16.  Each cell site has been allocated its own set-up
channel (control channel).
 The assigned forward set-up channel of each
cell site is used to page the mobile unit with
the same mobile station control message.
 No simulcast interference as same message is
transmitted using different cell sites.
 Draw back is that paging process is too long.
 When mobile unit responds to paging channel,
voice channel is assigned depending on signal
level and interference level.

17.  Increase channels by using narrow banding,
spread spectrum, or time division
 Increasing spatial spectrum frequency reuse
 Frequency management and channel
assignment
 Improving spectrum efficiency time
 Reducing load of invalid calls
 Voice storage service for no-answere calls
 Call forwarding
 Call waiting
 Queuing

18.  Self location scheme:- selects a set up channel
and makes mobile originating call.in this
method mobile unit prevents from sending the
necessary information regarding its location to
cell site.
 Autonomous registration
 Traffic load on set up channels
 Separation between access and paging

19.  Channel assignment refers to the allocation of
specific channels to cell sites and mobile units.
 Channel assignment to the cell sites-fixed
channel assignment.
 In fixed channel assignment, the channels are
usually assigned to the cell-site for relatively
long periods. Two types of channels are assigned
◦ Set-up channels
◦ Voice channels
◦ Supervisory audio tone

20.  Set-up channels: There are 21 channels assigned
each cell in a N = 4, 7, 12 cell reuse patterns.
 If set up channel antennas are Omni-directional,
then each cell only needs one set up channel.
 This leaves many unused set-up channels.
 However, the set up channels of blocks A and B are
adjacent to each other.
 In order to Avoid interference between block A and
B , setup channels in the neighbourhood of channel
333(block A) and channel 334(block B) are
preferably unused.

21.  This situation always occur in the morning, when cars
travel into the city, and at night, when the traffic
pattern reverses.
 If the traffic density is uniform, the unsymmetrical
mobile-unit antenna pattern does not affect the system
operation much.
 However, when the traffic becomes heavier as more
cars approach the city, the traffic pattern becomes
nonuniform and the sites closest to the city, cannot
receive the expected number of calls or handoffs in the
morning because of the mobile unit antenna patterns.
 At night, as the cars move out of the city, the cell sites
closest to the city would have a hard time handing off
calls to the sites away from the city.

22.  The traffic capacity at an omnidirectional cell
or a directional cell can be increased by using
the Underlay-Overlay arrangement.
 The underlay is the inner circle, and the
overlay is Outer ring.
 The transmitted powers of the voice channels
at the site are adjusted for these two areas.
 Then different voice frequencies are assigned
to each area.

23.  One Overlay and One under lay are shown in fig.
 Because of the sectorization in a directional cell,
the channel assignment has a different algorithm
in six regions.

24.  We assign the frequencies by a set of
channels or any part of a set or more than
one set of the total 21 sets.
 Borrowed frequency sets are used when
needed.
 On the basis of coverage prediction, we can
assign frequencies intelligently at one site or
at one sector without interfering with
adjacent co channel sectors or co channels.

25.  Channel Sharing. Channel sharing is a short-term
traffic-relief scheme.
 A scheme used for a seven-cell three-face system
is shown in Fig.
 There are 21 channel sets, with each set consisting
of about 16 channels. Figure . shows the channel
set numbers.
 When a cell needs more channels, the channels of
another face at the same cell site can be shared to
handle the short-term overload.

27.  Sharing always increases the trunking efficiency of
channels.
 Since we cannot allow adjacent channels to share
with the nominal channels in the same cell, channel
sets 4 and 5 cannot both be shared with channel
sets 12 and 18, as indicated by the grid mark.
 Many grid marks are indicated in Fig. for the same
reason.
 However, the upper subset of set 4 can be shared
with the lower subset of set 5 with no interference.

28.  In channel-sharing systems, the channel combiner
should be flexible in order to combine up to 32
channels in one face in real time.
 An alternative method is to install a standby
antenna.

29.  Channel borrowing is usually handled on a long-
term basis.
 The extent of borrowing more available channels
from other cells depends on the traffic density in
the area.
 Channel borrowing can be implemented from one
cell-site face to another face at the same cell site.

30.  In addition, the central cell site can borrow
channels from neighboring cells.
 The channel borrowing scheme is used
primarily for slowly-growing systems.
 It is often helpful in delaying cell splitting in
peak traffic areas.
 Since cell splitting is costly, it should be
implemented only as a last resort.

31.  Advantage of Sectorization.
 The total number of available channels can be
divided into sets (subgroups) depending on the
sectorization of the cell configuration:
 The 120◦-sector system, the 60◦-sector system,
and the 45◦-sector system.
 A seven-cell system usually uses three 120◦
sectors per cell, with the total number of channel
sets being 21.
 In certain locations and special situations, the
sector angle can be reduced (narrowed) in order to
assign more channels in one sector without
increasing neighboring-channel interference.

32.  Sectorization serves the same purpose as the
channel-borrowing scheme in delaying cell
splitting.
 In addition, channel coordination to avoid co
channel interference is much easier in sectorization
than in cell splitting.

33.  There are three basic types.
 1. The 120◦-sector cell is used for both
transmitting and receiving sectorization.
 Each sector has an assigned a number of
frequencies. Changing sectors during a call
requires handoffs.
 2. The 60◦-sector cell is used for both transmitting
and receiving sectorization.
 Changing sectors during a call requires handoffs.
 More handoffs are expected for a 60◦ sector than
a 120◦ sector in areas close to cell sites (close-in
areas).

34.  The 120◦- or 60◦-sector cell is used for receiving
sectorization only.
 In this case, the transmitting antenna is
omnidirectional.
 The number of channels in this cell is not
subdivided for each sector.
 Therefore, no handoffs are required when
changing sectors.
 This receiving-sectorization-only configuration
does not decrease interference or increase the D/R
ratio;
 it only allows for a more accurate decision
regarding handing off the calls to neighboring
cells.

35.  In actual cellular systems cell grids are seldom
uniform because of varying traffic conditions in
different areas and cell-site locations.
 Overlaid Cells. To permit the two groups to reuse
the channels in two different cell-reuse patterns of
the same size, an “underlaid” small cell is
sometimes established at the same cell site as the
large cell (see Fig. a).
 The “doughnut” (large) and “hole” (small) cells are
treated as two different cells.
 They are usually considered as “neighboring cells.”

37.  The use of either an omnidirectional antenna at
one site to create two sub ring areas or three
directional antennas to create six subareas is
illustrated in Fig.b.
 As seen in Fig. 12.4, a set of frequencies used in
an overlay area will differ from a set of frequencies
used in an underlay area in order to avoid
adjacent-channel and co channel interference.
 The channels assigned to one combiner—say, 16
channels—can be used for overlay, and another
combiner can be used for underlay.

39.  The antenna of a set-up channel is usually
omnidirectional.
 When an incoming call is received by the set-up
channel and its signal strength is higher than a
level L, the underlaid cell is assigned; otherwise,
the overlaid cell is assigned.
 The handoffs are implemented between the
underlaid and overlaid cells.
 In order to avoid the unnecessary handoffs, we
may choose two levels L1 and L2 and L1 > L2 as
shown in Fig.c.

41.  When a mobile signal is higher than a level L1
the call is handed off to the underlaid cell.
 When a signal is lower than a level L2 the call
is handed off to the overlaid cell.
 The channels assigned in the underlaid cell
have more protection against co channel
interference.

42.  Fixed Channel Algorithm.
 The fixed channel assignment (FCA) algorithm is
the most common algorithm adopted in many
cellular systems.
 In this algorithm, each cell assigns its own radio
channels to the vehicles within its cell.
 Dynamic Channel Assignment. In dynamic channel
assignment (DCA), no fixed channels are assigned
to each cell. Therefore, any channel in a composite
of N (312) radio channels can be assigned to the
mobile unit.
 This means that a channel is assigned directly to a
mobile unit. On the basis of overall system
performance, DCA can also be used during a call.

43.  Hybrid Channel Assignment.
 Hybrid channel assignment (HCA) is a combination
of FCA and DCA.
 A portion of the total frequency channels will use
FCA and the rest will use DCA.
 Borrowing Channel Assignment.
 Borrowing channel assignment (BCA) uses FCA as a
normal assignment condition.
 When all the fixed channels are occupied, then the
cell borrows channels from the neighboring cells.

44.  Forcible-Borrowing Channel Assignment.
 In forcible-borrowing channel assignment (FBCA),
if a channel is in operation and the situation
warrants it, channels must be borrowed from the
neighboring cells and at the same time, another
voice channel will be assigned to continue the call
in the neighboring cell.

45.  FBCA can also be applied while accounting for the
forcible borrowing of the channels within a fixed
channel set to reduce the chance of co channel
assignment in a reuse cell pattern.
 The FBCA algorithm is based on assigning a
channel dynamically but obeying the rule of reuse
distance.
 The distance between the two cells is reuse
distance, which is the minimum distance at which
no co channel interference would occur.

46.  Handoff is a process of automatically
changing frequencies as the mobile unit
moves into a different frequency zone so that
the conversation can be continued in a new
frequency zone without redialing.

47.  Why Handoffs?
 Handoff is needed in two situations where the cell
site receives weak signals from the mobile unit:
1. At the cell boundary, say,-100dBm, which is the
level for requesting a handoff.
2. When the mobile unit is reaching the signal
strength holes(gaps) within the cell site as shown in
below figure.

48.  There are two types of handoff:
i) That based on signal strength
ii) That based on carrier to interference ratio.
 In type 1, the signal strength threshold level for
handoff is -100dBm in noise-limited systems and
-95 dBm in interference-limited systems.
 In type 2, the value of C/I at the cell boundary for
handoff should be 18dB in order to have toll quality
voice.
 The location receiver at each cell site measures all
the signal strengths of all receivers at the cell site.
The received signal strength(RSS) is given by
RSS=C+I
Where C is the carrier signal power and
I is the Interference.

49.  At the cell site signal strength is always
monitored from a reverse voice channel.
 When the signal strength reaches a level of
handoff , then the cell site sends request to
the mobile telephone switching office (MTSO)
for a handoff on call.
 An intelligent decision can be made at the cell
site weather the handoff should have to take
place earlier or later.
 If an unnecessary handoff is requested, then
the decision was made too earlier.
 If a failure handoff occurs, then a decision
was made too late.

50.  The following approaches are used to make handoffs
successful and to eliminate all unnecessary handoffs.
 Suppose that -100dBm is a threshold level at the cell
boundary at which a handoff would be taken.
 In this scenario we must set up a new threshold level
which is higher than -100dBm.
 Let it be -100dBm+∆
 If ∆ is large unnecessary handoff may take place and
if it is too small time may not be enough for handoff,
therefore ∆ should be varied according to path loss
slope and level crossing rate(LCR) of signal strength.
 If the value of ∆ is 10dB, this mean that a level of
-90dBm is the new threshold level for requesting a
handoff.

52.  There are two circumstances where handoffs are
necessary but cannot be made:
1.When the mobile unit is located at a signal
strength hole with in a cell but not at the cell
boundary as shown in above figure.
2. When the mobile unit approaches a cell boundary
but no channels in the new cell are available.
 In case 1, the call must be kept in the old frequency
channel until it is dropped as the result of an
unaccepted signal level.
 In case 2, the new cell must reassign one of its
frequency channels with in a reasonably short
period or the call will be dropped.

53.  In may cases , a two handoff level algorithm
is used because to provide more opportunity
for successful handoff.
 A handoff could be delayed if there is no
available cell to take a call.
 A plot of signal strength with two request
handoff levels and a threshold level is shown
in figure.

54.  The above plot is recorded on the channel
received signal strength indicator(RSSI) which is
installed at each channel receiver at the cell site.
 When the signal strength drops below the first
handoff level a handoff request is initiated.
 If for some reasons the mobile unit is in a hole(a
weak spot in a cell) or a neighboring cell is busy,
the handoff will be requested periodically for
every 5 seconds.
 At the first handoff level, the handoff takes place
if the new signal is stronger(see case I in below
figure)
 However when the second handoff level is
reached, the call will be handed off with no
condition (see case II in below figure)

56.  The MTSO always handles the handoff call first
and the originating calls second.
 If no neighboring cells are available after the
second handoff level, the call continues until the
signal strength drops below the threshold level
and then the call is dropped.
 Advantage of delayed handoff:
 When the mobile units are moving randomly and
the terrain contour is uneven, the received signal
strength at the mobile unit fluctuates up and
down.
 In this situation if the mobile is in a hole for less
than 5 seconds, the delay in handoff can even
avoid the need for handoff.

57.  The other advantage of having a two-level
handoff algorithm is that it makes the handoff to
occur in a proper location and eliminates
possible interference in the system.
 In the above figure case I, the first level handoff
occurs between cell A and B. If it is not possible,
then we use the second level handoff.
 Case II also shows the second level handoff
occurs between cell A and C. This is because the
first level handoff cannot be implemented .

58.  A forced handoff is defined as the handoff
which would normally occur but is prevented
from happening or, a handoff that should not
occur but is forced to happen.
 This can be done by controlling a handoff or
creating a handoff.
 Controlling a handoff:
1. The cell site can assign a low handoff
threshold to keep a mobile unit in a cell
longer or high handoff threshold level to
request a handoff earlier.
2. The MTSO can also control a handoff by
making a handoff earlier or later, after
receiving a handoff request from a cell site.

59.  Creating a handoff:
 In this case, the cell site does not request a
handoff but the MTSO finds some cells are
too congested while others are not.MTSO can
request cell sites to create early handoffs for
those congested cells.
 In other words cell site has to follow the
MTSO order and increase the handoff
threshold to push the mobile units at the new
boundary and to handoff earlier.

60.  In normal Handoff procedure, the request for a handoff is
based on the signal strength at the cell site from the
reverse link.
 In the digital cellular system, the mobile receiver is
capable of monitoring the signal strength of setup
channels of neighboring cells while serving a call.
 For instance, in a TDMA system, one time slot is used for
serving a call, the rest of the time slots can be used to
monitor the signal strength of setup channels.
 When signal strength of its voice channel is weak, the
mobile unit can request a handoff and indicate to the
switching office that which neighboring cell is ready to
handover the call.
 Now the switching office has two pieces if information, the
signal strengths of both forward and reverse setup
channels of neighboring cell
 The switching office, therefore, has more intelligent
information to choose proper neighboring cell to handoff
the call.

61.  Some times call may be initiated in one
cellular system controlled by one MTSO and
enter another system controlled by another
MTSO before terminating.
 Inter system handoff can be defined as a call
handoff that can be transferred from one
system to second system so that the call can
be continued while the mobile unit enters the
second system.
 The software in the MTSO must be modified
to apply this situation.

63.  Consider the example shown in shown in
above figure.
 The car travels on a highway and the driver
originates the call in system A
 Car leaves cell site A of system A and enters
cell site B of system B
 When mobile unit signal becomes weak in cell
site A, MTSO A searches for a neighboring
cell site in the system and if it does not find
any one, MTSO A sends a handoff request to
MTSO B which makes a complete handoff
during the call conversation.

64.  “ The call is established “ means the call is
setup completely by the setup channel
 If there is a possibility of a call drop due to
no available voice channels, this is counted as
a ‘blocked call’ not a dropped call.
 If there is a possibility that call will drop due
to the poor signal of assigned voice channel,
that is considered as dropped call.
 If call is terminated before it is properly
terminated may be due to:
◦ Subscriber unit not functioning properly.
◦ Operating in vehicle
◦ User not having knowledge of best reception of
signal.

65.  One of major reason of dropped calls is
improper handoff, a proper timely handoff is
one of the procedures to reduce dropped
calls.
 During handoff between two cells due to an
imbalance of traffic between the two cell site
areas, it cannot accept the additional traffic
of the call then there is a chance of call
dropping. The dropping probability is defined
as the percentage of handoff attempts that
are denied because of insufficient resources
in the cell into which the mobile is moving.

66.  The number of dropped calls in cellular system
is dependent on the dropped call rate. The
dropped call rate is dependent on the following
factors:
• The channel capacity
• Level of traffic in the system (highly populated
areas such as metro cities and business area
save more chances of handoffs and so the
dropped call rate increases).
• Voice quality
• Probability that the signal below the receiver
threshold (∆)
• Probability that the signal below the specified
co-channel interference level (m)

67.  Channel capacity:
 Channel capacity is directly proportional to
bandwidth of the system. If bandwidth is more,
then more number of channels (users) can be
allotted.
 With the increase in channels, adjacent channel
interference also increases and so signal-to-
interference ratio decreases.
 This leads to poor signal quality and increased
dropped call rate.
 There is a relation between channel capacity, the
number of voice channels, and the signal to-noise
ratio as given below:
 The radio capacity
where N = total number of channels
S/I = required SIR ratio for designing a system.

68.  Level of traffi c in the system:
 Traffic intensity is the measurement of traffic
generated by a user during the busy hour (BH).
 The total number of voice calls originated or
terminated in a mobile during the BH is called voice
traffic arrival rate and voice traffic is generally
represented by the unit called Erlang.
 Erlang is defined as a voice call of one hour duration.
Each voice call is held for certain duration. The
average duration of all voice calls is called holding
time of a call. Similarly, departure rate can be
considered as “1/T”.
 If R represents the arrival rate of voice calls during a
BH (call/s)and T represents average holding time of a
call (in seconds), the total BH voice traffic is given by
RT. Then,

69. If we consider a whole cellular system, the general formula for
dropped call rate D will be given as